Recently, since the spread of the Internet drastically increased the traffic volumes of backbone communication systems, the early practical use of ultrahigh-speed optical communication systems exceeding 40 Gbps is expected. As a technique for realizing such an ultrahigh-speed optical communication system, a polarization demultiplexing technique has attracted attention.
The polarization demultiplexing technique is a technique that multiplexes two independent optical signals of which carrier waves are allocated in the same frequency band and polarization states are at right angles to each other in an optical transmitter and separates the above-mentioned two optical signals from received signals in an optical receiver, to thereby realize a double transmission rate.
On the contrary, since the symbol rate (baud rate) of the optical signal can be set to ½ and the operation speed of an electrical device can be reduced, device costs can be reduced in other words.
Hereinafter, reference will be made to the drawings to describe an operation of an optical communication system making use of the polarization demultiplexing technique (hereinafter, described as the optical communication system).
First, a description will be made of the process of generating optical signals in an optical transmitter which are used in an optical communication system. FIG. 10 shows a configuration example of an optical transmitter 10 in the optical communication system in the related art.
The optical transmitter 10 in the related art includes each of the blocks of a data source 101, a data partitioning section 102, a light source 103, a light branching section 104, an optical transmission section 105-1, an optical transmission section 105-2, and a polarization multiplexing section 106.
The data source 101 generates transmission data and then transmits the transmission data to the data partitioning section 102. Meanwhile, although the transmission data is typically supplied from other communication equipment connected to the optical transmitter, the optical transmitter 10 itself generates the transmission data for the purpose of simplicity herein.
The data partitioning section 102 dual-partitions (demultiplexes) the transmission data sent from the data source 101, and then transmits the respective resultants to the optical transmission section 105-1 and the optical transmission section 105-2.
As a method of partitioning the transmission data, it is possible to use various types of methods such as a partitioning method for each bit (bit interleave) or a partitioning method for each byte (byte interleave).
The light source 103 outputs laser light having a predetermined frequency to transmit the laser light to the light branching section 104. The light branching section 104 bifurcates the laser light transmitted from the light source 103, and transmits laser light having the same intensity to the optical transmission section 105-1 and the optical transmission section 105-2, respectively.
Although a method of supplying laser light from different light sources having the same light frequency and the same light intensity can also be applied to the optical transmission section 105-1 and the optical transmission section 105-2, it is preferable, in the invention, that carrier wave frequencies of the optical signals transmitted from the optical transmission section 105-1 and the optical transmission section 105-2, respectively, are identical to each other including line widths, and thus the configuration in which the above-mentioned single light source is bifurcated is recommended.
The optical transmission section 105-1 and the optical transmission section 105-2 perform an optical modulation on the basis of data sent from the data partitioning section 102, using the laser light sent from the light branching section 104 as a carrier wave. In the invention, the types of the optical modulation system do not matter. The optical signals generated by each of the optical transmission sections 105-1 and 105-2 are respectively sent to the polarization multiplexing section 106.
The polarization multiplexing section 106 multiplexes the optical signals sent from the optical transmission section 105-1 and the optical transmission section 105-2 so that the polarization states thereof are at right angles to each other, and then sends out the resultants to an optical transmission line 200. The optical signals generated by the optical transmitter 10 are propagated through the optical transmission line 200, and then are received by an optical receiver 30.
Next, a description will be made of the process of regenerating transmission data from the optical signals received by the optical receiver 30. FIG. 11 shows a configuration example of the optical receiver 30 in the optical communication system in the related art.
The optical receiver 30 includes an optical waveform equalization section 301, a polarization separation section 302, a light receiving section 303-1, a light receiving section 303-2, a data identification section 304-1, a data identification section 304-2, and a data multiplexing section 305.
The optical waveform equalization section 301 optically compensates a waveform distortion due to a wavelength dispersion received during the propagation of the optical signals through the optical transmission line 200, and then transmits the optical signals after the compensation to the polarization separation section 302.
The polarization separation section 302 separates an optical signal received by the optical waveform equalization section 301 into two optical signals generated by the optical transmission sections 105-1 and 105-2, and transmits each of the optical signals to a light receiving section 302-1 and a light receiving section 302-2.
The light receiving section 303-1 converts the optical signals sent by the polarization separation section 302 into electrical signals, and transmits the electrical signals to the data identification section 304-1. The same is true of the light receiving section 303-2.
The data identification section 304-1 converts the electrical signals sent from the light receiving section 303-1 into digital data on the basis of predetermined identification conditions appropriate to the optical modulation system, and transmits the digital data to the data multiplexing section 305. The same is true of the data identification section 304-2.
The data multiplexing section 305 regenerates original transmission data by multiplexing the digital data sent from the data identification section 304-1 and the data identification section 304-2.
However, as shown in FIG. 11(b), as the configuration of an optical receiver 31, a method of converting an optical signal received by the optical transmission line 200 into an electrical signal by the light receiving section 303 and then separating the electrical signals generated by the above-mentioned light receiving sections 303-1 and 303-2, respectively, from the converted electrical signal is also considered.
That is, FIG. 11(a) shows a configuration in which polarization separation is optically performed, and FIG. 11(b) shows a configuration in which polarization separation is electrically performed. As described above, the optical signal polarization-multiplexed in the optical transmitter is separated into the respective independent optical signals in the optical receiver, and then original transmission data is regenerated.
Meanwhile, an application of a wavelength dispersion measurement device (not shown) is filed that measures group velocity dispersion of an optical component located on the place from which an input and output terminal such as a transmission line optical fiber is separated, with a high degree of accuracy, without depending on residual intensity modulation.
In the wavelength dispersion measurement device, light emitted by a plurality of semiconductor lasers having different wavelengths is modulated by a plurality of light intensity modulators on the basis of a pulse of the electrical signal, and then is synthesized by an optical coupler.
The synthesized light passes through an optical component to be measured and is detected by a photodiode. The direct-current component contained in the electrical signal of the detected light and the intensity of the i/NT (i is an integer of 1 to N−1) frequency component are detected by a bandpass filter and a power meter.
Further, the average photocurrent flowing in the photodiode is measured by an ammeter. A dispersion value of the optical component to be measured is calculated from information of the intensity of the i/NT component, the average photocurrent, the pulse shape and the light source frequency which are measured (see, for example, Patent Document 1).
In addition, an application of a polarization multiplexing optical communication system (not shown) that stably separates polarization components in a simple configuration is also filed. In the polarization multiplexing optical communication system, an optical transmission section modulates and outputs any of the wavelength, the transmission timing, and the intensity of light which is a transmitted wave using a low-frequency signal transmitted from a low-frequency generator.
A polarization multiplexer synthesizes two modulated output beams in the polarization states which are at right angles to each other to generate a polarization multiplexing signal. A polarization splitter extracts and separates two polarization components which are at right angles to each other from the polarization multiplexing signal of which the polarization state is controlled by a polarization control section.
A bandpass filter extracts a component penetrating a pass band from an output signal of a light receiving section, and outputs the intensity of the component. A control circuit generates a feedback control signal for maximizing the ratio of a component of a low-frequency signal on the basis of the output intensity from the bandpass filter, and the polarization control section controls the polarization state of the polarization multiplexing signal using the feedback control signal (see, for example, Patent Document 2).